Effects of melengestrol acetate on the inflammatory response in heifers challenged with Mannheimia haemolytica

• Published in: Journal of animal science Vol. 85; no. 7; pp. 1770 - 1779
• Main Authors: Corrigan, M.E, Drouillard, J.S, Spire, M.F, Mosier, D.A, Minton, J.E, Higgins, J.J, Loe, E.R, Depenbusch, B.E, Fox, J.T
• Format: Journal Article
• Language: English
• Published: Savoy, IL American Society of Animal Science 01.07.2007; Am Soc Animal Sci
• Subjects: Acute-Phase Proteins - analysis; animal pathology; Animal productions; Animals; bacterial infections; Biological and medical sciences; biomarkers; Blood Glucose - analysis; Blood Urea Nitrogen; Cattle; cattle diseases; chronic diseases; Colony Count, Microbial - veterinary; dietary exposure; disease control; disease course; disease severity; Dose-Response Relationship, Drug; drug therapy; Female; Fibrinogen - analysis; Fundamental and applied biological sciences. Psychology; Haptoglobins - analysis; heifers; immune response; immunomodulators; inflammation; lesions (animal); Lung - immunology; Lung - microbiology; Lung - pathology; lungs; Mannheimia haemolytica; Mannheimia haemolytica - pathogenicity; melengestrol; Melengestrol Acetate - pharmacology; oral administration; Pneumonia of Calves, Enzootic - blood; Pneumonia of Calves, Enzootic - immunology; Pneumonia of Calves, Enzootic - pathology; protective effect; respiratory tract diseases; Terrestrial animal productions; Vertebrates; Vertebrata; Mammalia; melengestrol acetate; Bovine; Mannheimia haemolytica; Ox; Heifer; Artiodactyla; Acetate; Ungulata; Melengestrol
• ISSN: 0021-8812; 1525-3163
• DOI: 10.2527/jas.2006-396

Previous research from our laboratory has indicated that melengestrol acetate (MGA) added to the diet during the first 35 d after arrival in the feedlot improves growth rates and tends to reduce chronic respiratory disease in heifers naturally challenged with bovine respiratory disease. The current study was conducted to provide further insight into the possible immunomodulatory effects of MGA. Crossbred heifers (n = 48; 232 ± 5.5 kg of BW) were used in a randomized complete block design to determine the effects of MGA on lung pathology and markers of inflammation after Mannheimia haemolytica challenge. On d 0, cattle were blocked by BW and randomly assigned, within block, to diets (54% concentrate) that provided 0 or 0.5 mg of MGA per heifer daily for the duration of the experiment. Inoculum containing from 1.3 x 10⁹ to 1.7 x 10⁹ cfu of M. haemolytica (20 mL) was instilled at the bifurcation of the trachea on d 14. Blood samples were collected, clinical observations were made, and rectal temperatures were recorded for each animal at 0, 12, 24, 48, 72, 96, 120, and 138 h after inoculation. Heifers fed MGA had greater circulating concentrations of eosinophils and postchallenge concentrations of segmented neutrophils and white blood cells (P < 0.01) than controls, as well as elevated plasma protein, serum haptoglobin, and fibrinogen after M. haemolytica challenge (P < 0.01). Heifers fed MGA had lower plasma glucose (P < 0.01), greater plasma urea N (P = 0.02), and elevated respiratory indices (P < 0.01) compared with controls. Necropsies performed on d 6 after inoculation suggested that M. haemolytica challenge was relatively mild, because lesions were confined to a small portion of the lungs. On a 0 to 100 scale, average lung lesion scores were 3 and 1 for MGA-fed and control groups, respectively (P < 0.06). Heifers fed MGA before mild M. haemolytica challenge were more susceptible to infection, as evidenced by a greater number of heifers fed MGA exhibiting pulmonary lesions 138 h after inoculation than controls (14 out of 23 vs. 6 out of 24 for MGA and controls, respectively; P < 0.02).